1. Forming of High Strength Steels (HSS & A/UHSS) in the Automotive Industry
Dr. Taylan Altan, Professor & Director,
Eren Billur, Graduate Research Associate, Center for Precision Forming (CPF) and ERC/NSM
The Ohio State University, Columbus, OH /
- Prepared for -
AIDA-America, Dayton, OH
June 13-14, 2012

2. Outline
Introduction
Material Properties
Formability
Presses
Tribology
Springback
Summary

3. Potential advantages of HSS
Background
Potential advantages of HSS
Weight savings in auto bodies, 15% to 25%
Increase in crash resistance and safety.
[ “Structural Materials in Automotive Industries: Applications and Challenges”, GM R&D Center]

4. Introduction INCREASED STRENGTH DECREASED FORMABILITY
Ref: Sadagopan 2004

5. Engineering Stress-Strain Curve True Stress-Strain Curve = Flow stress
Material properties of HSS/AHSS/UHSS
Sheet properties (flow stress) determination
In common practice, the uniaxial tensile test is used to determine the properties/flow stress of sheet metal.
Tensile test does not emulate biaxial deformation conditions observed in stamping.
Due to early necking in tensile test, stress/strain data (flow stress) is available for small strains.
Necking begins
Engineering Stress-Strain Curve
True Stress-Strain Curve = Flow stress
In bulge test, flow stress over large strain can be obtained in biaxial stress state

6. Material Properties Flow Stress
n-value, as defined in Hollomon’s Equation:
is not constant.
Challenges:
Predicting uniform elongation,
Input of flow stress into FEA codes.
Ref: World Steel Association, 2009.

7. Material Properties 0.15 Determination of Flow Stress Tensile Test
Ref: Nasser et al 2010

8. Material Properties Determination of Flow Stress Bulge Test
Ref: Nasser et al 2010

9. Material properties of HSS/AHSS/UHSS
Schematic of viscous pressure bulge test setup at CPF (OSU)
Clamping force
Die diameter = 4 inches (~ 100 mm)
Die corner radius = 0.25 inch (~ 6 mm)
Bulge/ Dome height (h)
Pressurized medium
Initial Stage
Testing stage
Pressure (P)
Methodology to estimate material properties from VPB test, developed at CPF (OSU)
Measurement
Material properties
FEM based inverse technique
Pressure (P)
Dome height (h)
Flow stress
Anisotropy

10. Material properties of HSS/AHSS/UHSS
Bulge test samples
Before bursting
After bursting

11. Material Properties Determination of Flow Stress Bulge Test 0.49
Challenges:
Tensile test gives a very limited information,
Bulge test gives more reliable strain-stress data.
Ref: Nasser et al 2010

12. Material properties of HSS/AHSS/UHSS
Bulge test for quality control of incoming sheet material
Graph shows dome height comparison for SS 409 sheet material from eight different batches/coils [5 samples per batch].
Highest formability  G , Most consistent  F
Lower formability and inconsistent  H

13. Material properties of HSS/AHSS/UHSS
Drawability of AHSS steels
Cugy et al 2006
New generation AHSS steels (X-IP steel) have higher drawability than conventional mild steels.

14. Material properties of HSS/AHSS/UHSS
Loading and Unloading modulus of AHSS steels
[ULSAB-AVC Report/AISI Training Session document, 2002]
[Pervez et al 2005]
Springback (elastic recovery) of the formed part is proportional to stress.
Decrease in Young’s modulus with strain in AHSS steel results in higher springback.

15. Material Properties Apparent Modulus Variation Challenge:
changes with plastic
strain
Ref: Kardes et al 2010

16. Material Properties Inconsistency of Material Properties
AHSS are performance based grades.
TRIP 800
Challenges:
Strength, elongation, weldability may vary,
Material properties are inconsistent from supplier to supplier, even batch to batch.
Ref: Choi et al 2009.

17. Formability Local Failures Challenges:
Significant Stretching
Moderate Stretching and Bending
High Hole Expanding and Bending
Challenges:
Local failures do not correlate with n-value, R-value or elongation,
Materials has to be tested under various stress states.
Ref: Sung et al 2007; Dykeman et al 2009.

18. Formability Stretching (a) (b) (c) Challenges:
Higher Stretchability
Challenges:
Stretchability decreases with strength {(a) and (b)},
Inconsistency is present in stretching (c).
Ref: SSAB and Uddeholm 2008, Keeler and Ulnitz 2009, Dykeman et al 2009

19. Formability Bending Challenges: Bendability decreases with strength,
Higher Bendability
Elongation in bending does not correlate to elongation in tension test:
DP980 failed at 14% elongation in tensile, 40% elongation in bending.
Challenges:
Bendability decreases with strength,
Failure at bending cannot be predicted by tensile data.
Ref: World Steel Association 2009, Yan 2009

20. Formability Stretch Bending Challenge:
This type of fracture cannot be predicted using
conventional Forming Limit Curve (FLC).
DP980
B-pillar inner
DP780
Underbody structural part
Ref: Shi and Chen 2007

21. Formability Stretch Bendability
A suggested test method: Angular Stretch Bending (ASB)
Achievable heights of several steels: as strength increases,
stretch bendability decreases.
Ref: Sadagopan and Urban 2003, Wu et al 2006

22. Formability Deep Drawing Challenges:
Higher strength, results with less deep drawability.
Sidewall curls and local fractures are observed
Ref: SSAB and Uddeholm 2008, World Steel Association 2009

23. Formability Deep Drawing One solution to this problem is: Deep Drawing
Optimizing blankholder pressure, including multi-point cushion systems.
Ref: Palaniswamy and Altan 2006

24. Formability Flanging / Edge Stretching Hole Expansion Test
Cracked Sample
Ref: Sadagopan 2004, Sung et al 2007

25. Formability Flanging / Edge Stretching Effect of hole blanking
Worn Tool
Sharp Tool
Effect of hole blanking
Challenges:
Edge cracks cannot be predicted by FLC and are related to sheared edge quality,
Higher strength reduces the hole expansion ratio (HER),
HER gets even worse with worn tools
Ref: SSAB and Uddeholm 2008

26. Presses Required Load and Energy Challenge:
Due to higher strength, required press load and energy
are higher.
Ref: Keeler and Ulnitz 2009

27. Press and tooling for forming HSS/AHSS/UHSS
Press slide force and energy requirements
IISI, 2006
IISI, 2006
Presses with higher force and energy capacity required for forming AHSS steels due to its higher strength and higher strain hardening compared to mild steels

28. Press and tooling for forming HSS/AHSS/UHSS
Blank holder force requirements
Noel et al , 2005
Higher blank holding force required due to its higher strength and relatively thin gage used compared to conventional steel to form the part.
Hydraulic cylinders / Nitrogen gas springs built in the die to provide higher blank holder force required to form AHSS steels.

29. Press and tooling for forming HSS/AHSS/UHSS
Modification in transfer press for forming AHSS steel
Haller , 2006
Higher load in forming AHSS steels results in large tilting of transfer press slide.  reduction in part accuracy and press life.
Double slide transfer press with independent slide for lead press /drawing stage is preferred option.
Double action hydraulic press with cushion in press bed preferred for lead press  flexibility in choosing slide depending on die size.

30. Presses Reverse Load in Blanking Challenge: Due to higher strength,
blanking load (forward
tonnage) would be
higher, resulting in
higher reverse load.
Solutions:
Use stepped punches,
Keep the punches in good shape,
Reduce blanking speed,
Use hydraulic dampers.
Ref: Miles 2004, Boerger 2008

31. Press and tooling for forming HSS/AHSS/UHSS
Modification in blanking press for AHSS steel
Linkage drive kinematics for blanking press
Blanking force
Haller , 2006
Esher et al , 2004
Higher snap-through force in blanking AHSS steels  Detrimental to press life
Blanking press with linkage drive are introduced to reduce the velocity close to BDC to reduce snap-through forces.
Soft-shock – add on to the blanking press to reduce the impact force on the press and increase press life.

32. Press and tooling for forming HSS/AHSS/UHSS
Tooling for forming AHSS steel
Parting line of tool steel inserts
Haller , 2006
Esher et al , 2004
Conventional monoblock design from cast iron material not preferred for AHSS forming.
Cast iron tool with tool steel inserts are used for improved strength and wear resistance.
Cooling channels incorporated in dies to release heat quickly and increase stroking rate.

33. Lubrication and Friction
Challenges:
1) Higher contact pressure and higher temperature are detrimental for lubricants,
2) Temperature and pressure additives are needed
Ref: Kim et al 2009

34. Evaluation of Lubricants Using The Cup Drawing Test (CDT)
(in cooperation with HONDA and several lubricant companies)
Performance evaluation criteria (cups drawn to same depth):
Higher the Blank Holder Force (BHF) that can be applied without fracture in the drawn cup, better the lubrication condition
Smaller the flange perimeter, better the lubrication condition (lower coefficient of friction)

35. Tool Materials, Treatments, Coatings
Ref: Liljengren et al 2008

36. Tool Materials, Treatments, Coatings
Ref: Young et al 2009

37. Product development using HSS/AHSS/UHSS
Failure prediction in forming AHSS steel
Stoughton et al 2006
FLC based failure prediction not accurate – Need a better and reliable failure prediction criteria for die engineering and analysis

38. Springback
Higher springback
Ref: World Steel Association 2009

39. Springback Higher springback Springback compensation: Over forming,
Locally deforming / bottoming,
Stretching by higher forces.
Modeling of springback is a challenge:
Flow stress equations do not fit,
Unloading modulus may vary,
More Bauschinger effect is observed.
Ref: Sung et al 2007

40. Studies on forming of HSS/AHSS/UHSS
Studies are conducted by:
International Iron & Steel Institute (IISI) including programs such as ULSAB & ULSAC [
Auto-Steel Partnership (A-SP) [
American Iron and Steel Institute (AISI) [
All major steel companies, [Mittal/Usinor, U.S. Steel, ThyssenKrupp, Nippon Steel, POSCO, etc]
Analysis of springback in forming of a AHSS is conducted by CPF in cooperation with its member companies and universities in Germany and Sweden.

41. Summary
Use of AHSS will continue to increase in the automotive industry.
Low formability, high springback & high forces are primary concerns in forming AHSS.
Yield stress (flow stress), n-value & Young’s modulus change with deformation (strain).
Non uniformity in incoming material a concern in forming high strength steels  robust process design needed.
Bulge test , a better test to estimate the flow stress of AHSS sheet materials over large strain
Higher forming forces requires increased attention to tool specifications (Tool material, Heat treatment) & selection of die surface coatings. Die & process design requires more engineering.
In stamping of HSS, the requirements on stamping presses increase (higher forming forces, better controls, increased stiffness & off center loading capacity).
Prediction of potential failure locations and springback in die engineering and analysis not reliable  Need more investigation on the AHSS material behavior in different strain paths.

42. Summary Material Properties
Flow stress equations cannot be expressed in simple form (σ=kεn),
Flow stress data determined with tensile test is very limited (~ true strain),
Unloading modulus may vary with plastic strain,
Material properties are not consistent,

43. Summary
2. Formability
Local failures are common and these do not correlate to n-value, R-value or elongation,
Various tests (hole expansion, stretch bending, etc.) are required.
3. Presses
Higher load and energy required,
Higher reverse loads are observed in blanking.

44. Summary 4. Friction / Lubrication
Higher loads are temperatures observed,
Lubricants, tool materials, treatments and coatings have to be selected carefully.
5. Springback
Higher springback is observed,
Prediction of springback requires more sophisticated analyses

45. Questions / Comments
Contact information:
Taylan Altan, Professor and Director
Center for Precision Forming (CPF) /
The Ohio State University, Columbus, OH
Ph: (614)